1. Field of the Invention
The present invention relates generally to plant growing methods and containers, and particularly to methods and containers for air-root-pruning of growing plants intended to be transplanted.
2. Description of the Related Art
Landscape plants and other plants intended to be transplanted have, for some time, been grown out-of-doors in containers above ground. Container-grown plants can be grown at a faster rate than those grown in the field, and because the roots of container plants are not severed or otherwise disturbed when the plants are transplanted, such transplanting can take place at any time during the year, not just during the early spring as required with bare root or bagged nursery stock.
A variety of containers for growing plants intended to be transplanted have been developed and used previously, beginning with metal cans having drainage openings punched in the bottoms and progressing to present-day plastic containers. However, plants grown in conventional containers, especially woody plants, commonly experience problems such as spiral root growth and generally downward root orientation with little lateral root development.
In some containers having open bottoms, the roots are air-pruned when they reach the bottom, wherein the portion of each root extending into the air is killed by desiccation. However, the root tips are still at the bottom of the container rather than being laterally directed along the sides. It has been shown that the root growth of container-grown plants after being transplanted primarily involves the extension of roots which were present in the container at the time of transplanting as opposed to the development of new roots. Consequently, the number and the position of root tips present at the time of transplanting is very important to the rapid establishment, and frequently the ultimate survival, of container-grown plants.
In the typical root growth of a germinating seed in a container, a tap root forms first and quickly grows to the bottom of the container where it changes direction along the profile of the container. The tap root continues to grow unless it becomes physically trapped or exits the container and is air-pruned. Once the tap root growth is terminated, either by trapping or air pruning, the apical dominance of the root tip is lost and the development of secondary lateral roots occurs along the length of the tap root. The secondary lateral roots generally grow outwardly towards the sides of the container and slightly downwardly according to the geotropic nature of plant roots. When the lateral roots contact the sides of the container, they grow downwardly towards the bottom of the container where they may continue to follow the shape of the container, such as growing in a circular pattern in a circular container, unless their growth is terminated as a result of being physically trapped or air-pruned. When plants having this type of root growth are transplanted, the downwardly orientated roots at the bottom of the root growth provide little or no lateral anchorage. Also, the root tips may be at a depth in the soil where aeration is less favorable. In addition, the downwardly orientated roots are intermingled with each other and exert pressure on one another as the roots increase in diameter with age. This pressure restricts the normal flow of water and nutrients from the root tips to the top of the plant, as well as the downward flow of energy from the leaves.
Landscape plants have sometimes been grown in bottomless, square containers placed on wire surfaces, whereby roots reaching the bottom of the container are air-pruned. However, most of the root tips that are produced will undesirably end up at the bottom of the container. Also, the requirement of placing the containers on wire screen surfaces generally makes the practice uneconomical. Furthermore, such open bottom containers are not easily moved without spilling or losing growth medium contained therein.
More recently, containers have been developed for controlling spiral root growth whereby the root tips are physically trapped by surfaces in the container so that the roots are prevented from elongating, or the circling root tips are caused to pass through vertical openings in the sides of the container whereby the root tips are air-pruned. An example of the root tip trapping type of container is described in U.S. Pat. No. 4,442,628 (Whitcomb) issued Apr. 17, 1984. An example of a container having vertical side wall openings that air prune circling roots is described in U.S. Pat. No. 4,497,132 (Whitcomb) issued Feb. 5, 1985. While such root-tip-trapping and air-root-pruning containers have been utilized successfully, some spiral root growth can still take place and the development of lateral root tips has been less than optimum.
Therefore, there is a need for an improved container for growing plants intended to be transplanted. Such an improved container should prevent spiral root growth to an even greater degree than existing containers. It would be desirable if the improved container enhanced the development of lateral roots and branched root tips along and around the sides of the container.